A three-phase motor (such as a permanent magnet synchronous motor and induction motor) is used in automotive applications such as power steering systems. It is known to control the phase windings in a three-phase motor using pulse width modulated signals. The pulse width modulated signals are applied to an inverter or a series of switching devices that connect the phase windings of the motor to either a positive or negative (ground) terminal of the vehicle battery.
In particular, a series of switching devices are usually part of a switching circuit that drive the three-phase motor. A current sensor is used to help determine and track the voltages being applied to each phase winding of the motor. In the past, the switching circuit and current sensor have been mounted on a ceramic substrate. A series of wire bonds are used to interconnect the switching devices and components. The use of wire bonds and a ceramic substrate, however, is expensive and there is a need for less expensive materials and designs.
It would be beneficial to use a printed circuit board to mount and interconnect the switching devices, such as a printed circuit board made of an epoxy glass known as FR4. This would allow a manufacturer to use a Field Effect Transistor (FET) in the form of a surface mounted power device. It would also be beneficial to eliminate the need of wire bonds. This would reduce the cost of implementing the system by eliminating cycle time, factory automation equipment, and maintenance cost associated with traditional wire bond methods.
It has been found, however, that applying a system to a printed circuit board generates problems. For instance, a system that applies sinusoidal drive signals to a three-phase motor is subject to a phenomenon known as torque ripple. Torque ripple can be characterized as harmonics (distortion) in the sinusoidal motor drive voltages that are created when the voltage loss from phase to phase is not balanced. These torque ripple harmonics generate undesirable problems. For instance, consider a three-phase motor used in a power steering application in an automobile. A driver of the automobile will feel any torque ripple harmonics in the form of small but repetitive oscillations while turning the steering wheel. This is an undesirable condition to automobile drivers and a need exists for eliminating, or at least substantially reducing, the effect of torque ripple harmonics.
Accordingly, a need exists to reduce the cost of implementing a three-phase control system yet solves other problems associated with torque ripple harmonics. The present invention addresses ways to solve this need. In particular, the present invention solves the problem of torque ripple harmonics when applying the three-phase motor control circuitry in a printed circuit board layout. This is accomplished by providing a mechanism to optimize, or otherwise balance, the resistive and reactive impedances that occur when applying the three-phase motor control circuitry in a printed circuit board layout.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.